The invention relates to the field of communication systems. More particularly the present invention relates to carrier phase determination in frequency hopped communication systems.
Communication systems communicate signals using a variety of possible spectrum spreading and modulation methods. One such spread spectrum method is frequency hopping where a data signal modulates the hopped carrier signal that hops between a prescribed set of different carrier frequencies. One such data modulation method is a Gaussian minimum shift keying (GMSK). GMSK is a form of continuous phase modulation (CPM) that exhibits a very narrow spectral occupancy and a constant envelope, thus making it compatible with non-linear power amplifier operation without the concomitant spectral re-growth associated with non-constant envelope signals. These attributes render GMSK an attractive data modulation scheme in all high throughput Frequency Division Multiple Access (FDMA) satellite communication systems where only limited system bandwidth is available.
A GMSK signal is formed with a formatted data signal, such as a NRZ data stream of a series of data bits (each having a duration of T seconds) and passed through a Gaussian filter with bandwidth B Hz and an FM modulator usually with modulation index of 0.5. A GMSK signal can also be generated by passing the formatted data bit stream through the Gaussian filter and then through an integrator and then into a phase modulator. This method is an alternative and equivalent method of generating GMSK signals. The output of the Gaussian filter is a series of Gaussian filter pulse responses to the respective data bits of the input data stream that are passed through the frequency modulator usually having the 0.5 modulation index that frequency modulates the carrier signal. The response of the Gaussian filter to an input data bit typically extends over a predetermined number of bit periods creating overlapping superimposed component signal from a respective number of predetermined data bits. Hence, each of the resulting respective predetermined number of prior data Gaussian filter pulse responses contribute a signal component for modulating the carrier phase of the frequency modulated signal. As such, the continuous output of the FM modulator, at each bit time, depends upon a predetermined number of prior data bits, inputted into the Gaussian filter, and hence the Gaussian filter and FM modulator have memory represented by these overlapping superimposed signal components from the prior predetermined number of data bits. This memory over L prior data bit is known as intersymbol interference where the carrier phase during the present bit time depends on the bit pattern previous to that bit for a predetermined number of data bits. This memory length depends upon the Gaussian filter bandwidth bit period BT product. In typical implementations, the NRZ data stream is a series of bits that can be represented by data pulses having +/xe2x88x921V voltage levels. Each +1V or xe2x88x921V data pulse of a respective data bit contributes to a phase response that is accumulated over time within a frequency hop. The Gaussian filter provides pulse responses to the +1V or xe2x88x921V data pulses that are accumulated in the FM modulator. Each of the pulse responses are integrated in the FM modulator to provide a phase shift of xc2x1xcfx80/2 at steady state. The FM modulator output provides a modulo 2xcfx80 rendering of the accumulated phase response of all of the past xc2x1xcfx80/2 data bits from the start of the frequency hop. The FM modulator output, at anytime within the hop, contains an accumulated phase response based on all the previous bits from the start of the hop. The resulting modulo xcfx80/2 phase response is hence a continuous accumulated phase output reduced modulo 2xcfx80 as a function of all the prior data bits from the start of the hop.
The GMSK phase modulated signal transmitted by the GMSK transmitter arrives at an arbitrarily time at the GMSK receiver. The GMSK receiver generates a local carrier reference for coherent demodulation of the received GMSK signal. When a GMSK signal arrives arbitrarily in time at the receiver, there is a carrier phase difference between the received carrier signal and the locally generated carrier reference. The carrier phase is initially unknown due to the unknown propagation time between the transmitter and the receiver. Thus, the GMSK signal has a changing phase due to data modulation and a constant (but unknown) carrier phase due to unknown propagation time. Upon reception of the GMSK phase modulated carrier signal, the carrier phase must be firstly determined for demodulating the GMSK signal so that the resulting accumulative phase can be determined to then enable the reconstruction of the data stream at the receiver.
Hence, determining the carrier phase is essential in coherent communications so that the carrier phase modulated by the data stream can be determined to recover the data. It is desirable to determined the carrier phase rapidly and reliably for improved system performance. The carrier phase of a GMSK modulated frequency hopped signal or an on-off GMSK modulated frequency hopped signal must be estimated with the use of one or more synch words embedded in the data stream in each frequency hop. The synch word is placed at the beginning of the hop followed by data, and the synch word is known to the receiver so that the accumulative phase of the synch word is also known, so that the carrier phase can be determined at the beginning of the hop. Hence, when the synch word is at the beginning of the hop, there is no carrier phase ambiguity because the resulting carrier phase of the signal is due to the original carrier phase and the expected accumulated phase due to the modulation sequence used in the known synch word. This method of determining of the carrier phase requires that the known synch word be placed at the beginning of the hop follow by data. When the synch word is located at the start of a hop there is a vulnerability to synch word jamming preventing reliable reception and demodulation of the GMSK signal.
The current received signal phase is the sum of a carrier phase and the accumulated data phase of the previous data channel bits. The pulse response from the Gaussian filter extends over 1/(BT) bit periods represented as an integer for denoting the intersymbol interference memory length. For GMSK signals with a memory of L channel bits, the preceding Lxe2x88x921 channel bits will establish the current data phase. If the prior Lxe2x88x921 channel bits were known and communicated in advance of the synch word, it is possible to determine the carrier phase at the beginning of the hop from the known prior data bits inducing expected data phase changes prior to the synch word. However, true unknown data is not known a priori. Hence, if the synch word or words of known data bits are placed at any other location within the hop beyond the start of the hop, the current phase at the receiver will be affected by both the unknown carrier phase and the unknown data phase resulting from the channel bits prior to the synch word. When the synch word is not placed at the beginning of the hop so that random data precedes the synch word, both carrier phase and data phase are unknown, and the carrier phase becomes ambiguous, prohibiting carrier phase determination and thereby prohibiting data demodulation. It thus appears that the synch word cannot be placed anywhere other than at the beginning of the hop. Hence, prior GMSK frequency hop signaling methods do not enable coherent demodulation of random channel data bits at the beginning of the hop prior to the first synch word. Interference can corrupt the reception of signals, and when interference affects the synch word at the beginning of a hop, the entire hop data will not be recovered. Hence, it is desirable to place the synch word or words arbitrarily within a hop, and still demodulate the channel bits. These and other disadvantages are solved or reduced using the present invention.
An object of the invention is to provide a method for determining the carrier phase (of the hopped carrier) modulo 2xcfx80 of Gaussian minimum shift keyed (GMSK) frequency hopped signal having a synch word arbitrarily positioned within a frequency hop.
Another object of the invention is to demodulate first and second data portions of a frequency hop having a synch word arbitrarily placed between the first and second data portions within a frequency hop signal.
A further object of the invention is to demodulate first, second, and third data portions of a frequency hop having a first synch word position between the first and second data portions, and having a second synch word positioned between the second and third data portion all within a frequency hop.
Another object of the invention is to demodulate a plurality of data portions of a frequency hop signal having a synch word between each pair of consecutive portions of the plurality of data portions all within a frequency hop.
Yet another object of the invention is to demodulate first and second random data portions by determining the unknown carrier phase of the carrier signal which has a known synch word and zeroing and guard bits arbitrarily placed between the first and second data portions with the zeroing and guard bits located just prior to the synch word, and contiguous to it, to generate a known data phase for determining the carrier phase for demodulating the second data portion and then demodulating the first data portion.
The invention is directed to a GMSK data modulated frequency hopped communication method using zeroing channel bits and channel guard bits just prior to a known synch word arbitrarily placed between the first and second data portions of a frequency hop for estimating the carrier phase of the frequency hopped signals. A frequency hopped communication system includes a transmitter transmitting GMSK frequency hopped signals and a receiver processing the GMSK frequency hopped signals. The transmitter generates GMSK continuous phase signals. The receiver demodulates the GMSK frequency hopped signals using coherent demodulation by estimating the phase of the carrier on each hop. The carrier phase is constant over a frequency hop. The transmitter inserts the zeroing channel bits and channel guard bits during transmission of each frequency hop, and the receiver detects the zeroing channel bits and channel guard bits for determining the carrier phase which is needed to demodulate each frequency hop. The synch words are positioned arbitrarily within each hop. The zeroing channel bits and channel guard bits are located just prior to the synch word and are used to establish zero cumulative phase modulo 2xcfx80 due to the first data position that is located in advance of the synch word so that the carrier phase can be determined by subtracting the synch word induced phase from the phase estimated at the end of the synch word. Once the carrier phase is known, the second data portion of the frequency hop can then be demodulated, and during a second demodulation pass, the first data portion can then be demodulated.
One zeroing phase channel bit is used for on-off keying GMSK (OOKGMSK) frequency hopped signals. Two zeroing phase channel tri-bits are used for GMSK frequency hopped signals. The zeroing tri-bits that are used for GMSK are defined as binary ones, binary zeros and true zeros. The carrier phase can increase by xcfx80/2 radians for a binary one, decrease by xcfx80/2 radians for a binary zero, or not change for a true zero. In other words, three states must be available for the zeroing bits for GMSK frequency hopped signal. However, only the usual two bits, binary zero and binary one, are needed for the OOKGMSK frequency hopped signal. In either case, Lxe2x88x921 channel guard bits are used in the frequency hopped signals where L=1/BT and is the Gaussian filter intersymbol interference memory of the GMSK modulation, where B is the 3 dB bandwidth of the GMSK Gaussian filter, and where T is the channel bit duration. Preferably, the Lxe2x88x921 guard channel bits are all true zero bits.
The communication method requires for the OOKGMSK frequency hopped signal one zeroing bit plus Lxe2x88x921 overhead guard bits where L is the filter memory of the GMSK signal corresponding to channel bit periods. For the GMSK frequency hopped signal, two tri-bit zeroing bits are needed plus Lxe2x88x921 guard bits that are true zero. Synch word, or words, may be placed anywhere in a GMSK modulated hop as long as the zeroing bits are Lxe2x88x921 channel guard bits are located just prior to the synch word or words. The carrier phase for coherent demodulation can be unambiguously estimated and the data can then thereafter be demodulated. The zeroing bit and guard bits establish a known data phase at the beginning of each synch word so that the carrier phase can be determined for demodulating the data portions of the hop for reconstructing the estimate of the transmitted data at the receiver. These and other advantages will become more apparent from the following detailed description of the preferred embodiment.